Exposure-Dependent Increases in IL-1β, Substance P, CTGF, and Tendinosis in Flexor Digitorum Tendons with Upper Extremity Repetitive Strain Injury

Upper extremity tendinopathies are associated with performance of forceful repetitive tasks We used our rat model of repetitive strain injury to study changes induced in forelimb flexor digitorum tendons Rats were trained to perform a high repetition high force (HRHF) handle-pulling task (12 reaches/min at 60 + 5% maximum pulling force [MPF]), or a low repetition negligible force (LRNF) reaching and food retrieval task (three reaches/min at 5 +/- 5% MPF), for 2 h/day in 30 mm sessions, 3 days/week for 3-12 weeks Forelimb grip strength was tested Flexor digitorum tendons were examined at midtendon at the level of the carpal tunnel for interleukin (IL)-1 beta, neutrophil, and macrophage influx, Substance P, connective tissue growth factor (CTGF), and periostin-like factor (PLF) immunoexpression, and histopathological changes. In HRHF rats, grip strength progressively decreased, while IL-1 beta levels progressively increased in the flexor digitorum peritendon (para- and epitendon combined) and endotendon with task performance Macrophage invasion was evident in week 6 and 12 HRHF peritendon but not endotendon Also in HRHF rats; Substance P immunoexpression increased in week 12 peritendon as did CTGF- and PLF-immunopositive fibroblasts, the increased fibroblasts contributing greatly to peritendon thickening Endotendon collagen disorganization was evident in week 12 HRHF tendons LRNF tendons did riot differ from controls, even at 12 weeks Thus, we observed exposure-dependent: changes in flexor digitorum tendons within the carpal tunnel. including increased inflammation, nociceptor-related neuropeptide immunoexpression, and fibrotic histopathology, changes associated with grip strength decline (C) 2009 Orthopaedic Research Society Published by Wiley Periodicals, Inc J Orthop Res 28 298-307, 2010